Orthogonal frequency division multiple access (OFDMA) modulation is well known to have a high peak-to-average power ratio (PAPR). A high PAPR reduces the power efficiency of a transmitter power amplifier (PA) and increases PA back-off, the latter of which reduces the uplink link budget. Therefore, it is desirable to control the PAPR for uplink transmission.
The PAPR reduction for OFDMA modulation has been studied extensively, with several possible solutions emerging. Tone reservation (TR) is one of the promising techniques. With TR, the system reserves a set of sub-carriers, or tones, for PAPR reduction. The reserved tones are not used for data transmission. Instead, when one signal has high PAPR, a compensating sequence is transmitted, using the reserved tones, to reduce the PAPR of the signal.
However, the conventional TR approaches have some drawbacks. The reserved tones may not be used for data transmission, which increases the system overhead. Further, there exist only a limited and fixed number of available reserved tones. These concerns limit the potential PAPR reduction benefit using the TR technique.
WiMAX (i.e., the Worldwide interoperability for Microwave Access), as currently defined by IEEE (Institute of Electrical and Electronics Engineers) 802.16-series specification, uses OFDMA in the uplink due to its generally outstanding properties and in order to simplify the overall standard by maintaining similarity with the downlink signal. In contrast, the long-term evolution (LTE) effort in the third generation partnership project, known as 3GPP, is contemplating the use of SC-FDMA (Single Carrier-Frequency Division Multiple Access) in the uplink while maintaining a choice of OFDMA for the downlink. The justification for choosing SC-FDMA over OFDMA in the LTE uplink is due to the PAPR issue.
Thus, there is a continuing need for an uplink transmission scheme that overcomes the shortcomings of the prior art.